Generally, a capacitor is a general electrical part and is widely used for as a power supply circuit, a noise filter and a digital circuit component in various electric/electronic parts. Capacitors are roughly classified into electrolytic capacitors and other capacitors such as ceramic capacitors, film capacitors, etc.
Various types of electrolytic capacitors are used at present and examples thereof include aluminum electrolytic capacitors, wet tantalum electrolytic capacitors and the like. It is an aluminum electrolytic capacitor from which a particularly excellent function is expected in the present invention. Therefore, the present invention will now be described with reference to this kind of an electrolytic capacitor. The term "electrolytic capacitor" used herein refers to an aluminum electrolytic capacitor unless otherwise stated.
A conventional aluminum electrolytic capacitor can be produced typically by using an anode foil, which is made by etching a high-purity aluminum foil to thereby increase its surface area, and anodizing the surface of the aluminum foil to provide an oxide film, and a cathode foil whose surface has only been etched. The resulting anode foil and cathode foil are disposed opposite each other and an element with a wound structure is made by interposing a separator (release paper) between those foils and then the element is impregnated with an electrolytic solution. The element impregnated with the electrolytic solution is contained in a case (generally made of aluminum), which is then sealed with an elastic sealant, thus completing an electrolytic capacitor. Electrolytic capacitors also include electrolytic capacitors other than those with a wound structure.
In the above-described electrolytic capacitor, the characteristics of the electrolytic solution may be a large factor which decides the performance of the electrolytic capacitor. With the size reduction of the electrolytic capacitor, an anode foil or cathode foil having a large surface area produced by etching has been used and the resistivity of the capacitor has recently increased. Therefore, an electrolytic solution having a low resistivity (specific resistance) and thus high conductivity is required as an electrolytic solution to be used in the electrolytic capacitor.
A conventional electrolytic solution for use in an electrolytic capacitor is generally prepared by dissolving, as an electrolyte, a carboxylic acid such as adipic acid, benzoic acid, etc. or an ammonium salt thereof into a solvent prepared by adding about 10% by weight or less of water to ethylene glycol (EG) as a principal solvent. Such an electrolytic solution has a specific resistance of about 1.5 .OMEGA..multidot.m (150 .OMEGA..multidot.cm).
On the other hand, the capacitor is required to have a low impedance (Z) to sufficiently exert the performance thereof. The impedance is decided by various factors and, for example, it is reduced when the electrode area of the capacitor increases. Therefore, an attempt to reduce the impedance is made as a matter of course in case of a large-sized capacitor. An attempt to reduce the impedance by improving a separator has also been made. However, the specific resistance of the electrolytic solution is a large controlling factor, particularly in a small-sized capacitor.
A lower-specific resistance electrolytic solution using an aprotic organic solvent such as GBL (.gamma.-butyrolactone) has recently been developed (see, Japanese Unexamined Patent Publication (Kokai) Nos. 62-145713, 62-145714 and 62-145715). However, the capacitor using this aprotic electrolytic solution is by far inferior in impedance to a solid capacitor using an electronic conductor having a specific resistance of 1.0 .OMEGA..multidot.cm or less.
The aluminum electrolytic capacitor has poor low-temperature stability because of use of an electrolytic solution, and a ratio of an impedance at -40.degree. C. to that at 20.degree. C. (100 kHz), Z (-40.degree. C.)/Z (20.degree. C.), is as large as about 40 at present. Under these circumstances, it is now required to provide an aluminum electrolytic capacitor which has a low impedance and excellent low-temperature stability.
Further, water used as portion of the solvent in the electrolytic solution of the aluminum electrolytic capacitor is a chemically active substance to aluminum constituting the anode foil or cathode foil. Accordingly, there is a problem that water reacts with the anode foil or cathode foil, thereby to generate a hydrogen gas and to drastically deteriorate the performance as a capacitor.
To solve a problem such as generation of hydrogen gas found in a load life test of the electrolytic capacitor, a trial of absorbing the generated hydrogen gas has hitherto been made. For example, Japanese Examined Patent Publication (Kokoku) No. 59-15374 discloses an electrolytic solution, for use in operation of an electrolytic capacitor, produced by adding a carboxylic acid and an ammonium salt of the carboxylic acid to a solvent having added thereto 5 to 20% by weight of water, thereby to prepare a buffer solution and further adding 0.05 to 3% by weight of p-nitrophenol to the buffer solution. When using this electrolytic solution, there can be provided an electrolytic capacitor wherein low-temperature stability and a working life characteristics are improved by inhibiting the occurrence of the boehmite reaction and generation of the hydrogen gas.
Japanese Unexamined Patent Publication (Kokai) No. 63-14862 also discloses an electrolytic solution for use in the operation of an electrolytic capacitor capable of exhibiting an excellent corrosion preventing function against washing with a halogenated hydrocarbon, which is produced by adding o-nitroanisole to an electrolytic solution prepared by dissolving various organic acids, inorganic acids or salts thereof in a solvent composed exclusively of ethylene glycol. This publication describes that o-nitroanisole used as a corrosion inhibitor has a hydrogen gas absorption function, that is, a function of absorbing a hydrogen gas generated from the interior during the use of the electrolytic capacitor, thereby making it possible to inhibit an accident of safety-vent operation and a change in capacitance.
However, it has been found, as a result of the present inventors' study, that p-nitrophenol or o-nitroanisole can exhibit an initial hydrogen absorption function in the case of a conventionally used electrolytic solution of low water concentration for use in operation of an electrolytic capacitor, but cannot exhibit and maintain a satisfactory hydrogen gas absorption function when a content of water is 20% by weight or more based on the solvent in the electrolytic solution or when the electrolytic capacitor is operated under high temperature conditions for a long period of time.